Knotmeter for small craft

ABSTRACT

A regulated source of power is used to set up a magnetic field in the water around a rodmeter. The output is fed to an amplifier which in turn is connected to a ring demodulator which produces a pulsating D.C. voltage. A capacitor smoothes the D.C. output and feeds it to an integrating digital voltmeter to provide a display which is proportional to the average speed of the craft through the water.

United States Patent Springston, Jr. et al.

[54] KNOTMETER FOR SMALL CRAFT [72] inventors: George B. Springston,Jr., Bethesda; Samuel L. Thomas, Gaithersburg; fled B. Miller,Rockville, all of Md.

[73] Assignee: The United States of America as represented bytheSecretaryoi' the Navy [22] Filed: Nov. 30, 1970 [21] Appl. No.:93,673

[52] U.S. Cl. ..73/181, 73/194 EM [51] Int. Cl ..G0lc 21/00 [58] FieldoiSear-ch ..73/181, 194; 324/99R [56] References Cited UNITED STATESPATENTS 3,362,221 1/1968 Eller et al ..73/194 EM 51 July 18, 1972Hutcheon et al ..73/ 194 EM Wasserman ..329/99 R Soller et al ..73/181Primary Examiner-Donald O. Woodie] Attorney-R. S. Sciascia, Q. E. Hodgesand R. M. Wohlfarth ABSTRACT A regulated source of power is used to setup a magnetic field in the water around a rodmeter. The output is fed toan amplifier which in turn is connected to a ring demodulator whichproduces a pulsating DC. voltage. A capacitor smoothes the DC. outputand feeds it to an integrating digital voltmeter to provide a displaywhich is proportional to the average speed of the craft through thewater.

2 Claim, 3 Drawing Figures PATENTED JUL 1 8 m2 sum 1 0F 3 FIG.

IN V EN TORS GEORGE E. SPRl/VGS TON, Jr.

SAMUEL L. THOMAS A T TORNE Y5 PATENIED JUL 1 8 I972 SHEET 3 UF 3 I O0.DO. 5?

00. O0. a i v Q 1 INVENTORS GEORGE E. SPR/NGSTOMJI:

SAMUEL L. THOMAS KNOTMETER FOR SMALL CRAFT The invention describedherein may be manufactured and used by or for the Government of theUnited States of America for Governmental purposes without the paymentof any royalties thereon or therefor.

BACKGROUND OF THE INVENTION 1. Field of the Invention The measurement ofship's speed through the water during hydrodynamic evaluation trialsproduces data which are useful in correlating other measurements such asroll, pitch, heave, thrust, torque, horsepower, and shaft rpm. For manyyears electromagnetic log equipment has been used as standard equipmenton most large high-speed military ships. As a speed-measurement systemfor general operation and navigation of ships, its performance isadequate. However, a small unit that can be used in smaller craft isrequired, but with the same need for accuracy found in the largeshipboard units. Also, in order to be readily usable to evaluate seatrails, the data should be displayed to the operator of the craft and berecorded or presented at the same time in a form that can be handled bya computer.

Small craft are susceptable to various sea states since they experiencea great amount of relative up and down motion causing fluctuation in thespeed information data. To alleviate this difficulty, a time-averagedvalue of the speed data can be provided rather than the instantaneousvalue given by a standard Navy log system.

2. Description of the Prior Art Present speed-measurement devices fallinto two categories; large shipboard units with acceptable accuracylevels, and small units capable of mounting on small boats but generallylacking accuracy. In addition, neither category of units provide aspeed-averaging capability which is critical in roughwater conditions.

Further, most speed-measuring devices do not provide a permanent recordof the data, and of those that do, the data are not compatible withmodern electronic computers. Also, error can be introduced into the dataif other electrical equipment on board the craft operates at the samefrequency as the rodmeter, thereby setting up electrical fields withinterference therefrom causing the error.

SUMMARY OF THE INVENTION The standard Navy rodmeter has proven to be anadequate sensor when used with an all semiconductor electronic system.The invention eliminates the use of a null-balance servosystem therebyreducing the size and weight of the unit but without sacrificing the theaccuracy or resolution of the standard system. The power source thatprovides excitation for the standard sensor is 70 Hz. in the preferredembodiment of the invention, to preclude interference from shipboardequipment at 60 Hz. The invention incorporates amplification anddemodulation circuitry with inherently high interference rejection, withthe data output suitable for recording on magnetic tape which iscompatible with modern electronic computers. Further, the gain of theamplifiers is adjustable so that the output can be used to provide adigital display of the craft speed directly in knots for the operator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of thesensor used in the invention with parts cutaway for clarity.

FIG. 2 is a block diagram of the system embodying the invention.

FIG. 3 is a schematic diagram of the system embodying the invention.

DESCRIPTION OF THE INVENTION Referring now to the drawings, the rodmeteror sensor is shown in FIG. 1 which is connected into the systemembodying the invention as shown in FIG. 2.

The sensor 10 has a streamlined body or housing 12 in which is mounted acoil 14. A convenient method of mounting is to pot the coil 14 withinthe housing 12. A cable 16 connects the coil 14 to its power source. Apair of stainless steel buttons 18 are the pickups to provide the inputto the system. A cable 20 connects the buttons 18 into the system. Thedirection of movement of the sensor 10 through the water is indicated bythe arrow A, with the alternating magnetic field set up by theexcitation of the coil 14 being shown by the dashed lines B. Themagnetic field B causes a voltage to be induced in a water circuit whichis shown at C which is measured across the buttons 18. The voltage is inphase with the current used to excite the coil but is approximately outof phase with the excitation voltage. The voltage measured across thebuttons 18 is directly proportional to the motion of the sensor throughthe water. This is well known in the art as shown in the U.S. Pats. toSoller et al., No. 3,1 14,260 and Snyder et al., No. 2,969,673.

The connection of the rodmeter 10 into the system is shown in FIG. 2wherein an oscillator 22 is used to provide a 70 Hz signal of about 5volts rms. The signal is then fed to a regulator 24 and an amplifier 26to produce an amplitude output of approximately 25 volts rms with apower output of approximately 20 watts to drive the coil 14.

The output of amplifier 26 and regulator 24 is also fed to a phasingnetwork 28 so that the output voltage is in phase with the voltage fromthe pickup buttons 18. This phased voltage is then used as the balancingand calibration voltage in the balance and calibration circuitry 30 andas in injected carrier for the ring demodulator 32. The voltage from thepickup buttons 18 and the voltage from the balance and calibrationcircuitry 30 are then applied to the three-stage amplifier 34. Thecomponent of the amplified signal from 34 which is in phase with theinjected carrier from the phasing network 28 is then rectified by thering demodulator 32. The ring demodulator has excellent rejectioncharacteristics for spurious signals, which is further enhanced when thesignal is averaged over a IO-second period by an integrating digitalvoltmeter 36. The use of the voltmeter 36 automatically results inbinary-coded decimal data which can either be recorded on tape alongwith other parameters or fed into a computer.

Referring now to the schematic of the system in FIG. 3, the excitationor carrier for the system is generated by a crystalcontrolled 70 Hzoscillator 22, which can be a commercially available type, such as theConnor-Winfield Type S21ON. The signal generated by oscillator 22 isabout 5 volts ms and is attenuated by a resistive network consisting ofpotentiometer 38 and resistor 40. The attenuated signal is fed to thesource of a junction field-effect transistor 42 which functions as avoltagevariable resistor. This transistor is the gain-controllingelement in operational amplifier 44 of the regulator 24, operationalamplifier 44 can be the commercially available Philbrick Type P65AU. Thedrain of the fieldeffect transistor 42 is connected to the negativeinput of operational amplifier 44 whose output forms the drive signalfor power amplifier 24, for example the commercially available KnightModel 3221T. The power amplifier 24 is used to raise the amplitude ofthe output of regulator amplifier 44 from approximately 0.15 volt rms to25 volts ms with a power output of approximately 20 watts to drive thesensor coil 14. The power for amplifier 24 comes directly from a volt ACpower source. Power for the remainder of the system is supplied by twoDC. power supply modules 46 producing 1:15 volts with respect to ground.

The output of power amplifier 24 is also fed to a rectifierfilternetwork, and a portion of the peak value of the output is fed back tothe gate of field-effect transistor 42 through an appropriate feedbackloop 43 in such polarity that an increase in the sensor drive voltagewill decrease the overall gain of amplifier 44. A closed-loop system isthus formed to hold the sensor drive voltage constant. A potentiometer48 controls the amount of feedback through the loop 43 and is used toadjust the sensor drive voltage to 25.000 volts rms. The closed-loopregulator thus described is capable of holding the output to within imillivolts ms of the set value.

The output voltage of power amplifier 24 is phased by a resistor 50, acapacitor 52, and the impedance of a transformer 54 such that itsvoltage is in phase with the voltage from the pickup buttons 18 of thesensor. This phased voltage is then used as an unjected carrier, viatransformer 54, and as the balancing and calibration voltages throughresistors 56 and 60, and potentiometers 58 and 62, and transformer 64.

The voltage from the pickup buttons 18 of the sensor 10 is applied tothe differential input of the first stage 66 of amplifier 34. This firststage 66, and its appropriate feedback loops 67 and 68 are chosen toprovide a gain of 10 with an input impedance of 200K ohms. Apotentiometer 69, in the feedback loop 68, is the common-mode rejectionbalance and when properly adjusted this stage will provide better than90 decibels of common-mode rejection. A voltage divider, consisting ofresistor 56 and potentiometer 58, provides a balance voltage which issummed into the input of the first stage 66 of amplifier 34 to cancelany stray carrier-frequency voltages appearing at the pickup buttons 18at zero speed. Likewise, another voltage divider, consisting of resistor60 and potentiometer 62 provides a calibration voltage which may besummed into the input of the first stage 66 to provide a knowncalibration step.

The second stage 78, of amplifier 34, and its appropriate feedback loop79 are chosen to provide a gain which is adjustable from 2 to 4 by acalibrated dial. Thus, the gain can be reset to any previously knownvalue. The output of the system can then be adjusted by this dial toread directly in knots.

The third stage 80, of amplifier 34, and its appropriate feedback loop81 provide a fixed gain of 5. The output of this stage is capacitivelycoupled to the signal input of a transformer 82. Capacitive coupling isused in order that any DC offset ap pearing at the output of the thirdstage 80 will not cause heavy current to flow and to saturate theamplifier or transformer 82.

The component of the amplified signal, either in phase or 180 out ofphase with the injected carrier, is rectified by the ring demodulator 32consisting of diodes 84, 86, 88, and 90 and resistors 100, 102, 104, and106. The input signals to the ring demodulator 32 are by means oftransformers 54 and 82 which have static shields between the windings.These transformers are chosen with static shields between the windingsto avoid any capacitive coupling within the transformers. Further,inorder to secure a highly accurate reference point for each drive inputto the ring demodulator, the transformer center taps are not used.Instead, precision resistors 92, 94, 96, and 98 are used to securereference points which are more accurate than the usual transformercenter tap.

The action of the ring demodulator 32 is as follows. The injectedcarrier appears across the resistors 96 and 98. This carrier voltagefirst causes the junction of resistors 100 and R02 to be held at groundpotential while the junction of resistors 104 and 106 floats freely,then on the next half cycle of the injected carrier, the opposite actionis obtained. Inspection of the diagram shows that the signal acrossresistors 92 and 94, which is either in phase or 180 out of phase withthe injected carrier, is rectified and appears at half amplitude at thejunction of resistors 92 and 94. The amplitude of the injected carrieris chosen to be large in comparison with the expected signal. The diodesthus become switches and the conduction characteristics of the diodesare essentially removed from the operation of the circuit. Ideally, thedemodulator 32 would be operated with the injected carrier transformedinto a square wave. Using a large injected carrier approaches the idealcondition without the complications of added circuitry. It should benoted that the circuit only responds to the carrier frequency and to oddharmonics thereof wherein response to the third harmonic is about 30percent of the fundamental with subsequent odd harmonics being evenfurther down. The response of the demodulator 32 to signals generated bysources other than the oscillator 22 which supplies the inected carrier,is very low as a result of the necessi y for a constant phaserelationship.

In addition, to the excellent rejection of spurious signals provided bythe demodulator, further conditioning is provided by utilizing anintegrating digital voltmeter 36 such as manufactured by Non-LinearSystems, Inc., which averages the signal for a l0-second time period.This voltmeter contains an extremely linear voltage-to-frequencyconverter. This frequency is counted for 10 seconds, and thus providesan average value of the input taken over that period. The main advantageof this type of analog-to-digital conversion is that it providesaccurate readings in the presence of large values of superimposed noisebecause the input is integrated over the sampling period. A true averageof the input voltage is produced.

As mentioned above, the use of a digital voltmeter automatically resultsin binary-coded decimal data. The data either can be recorded on tapealong with other parameters being measured during a computer for furtherprocessing or use with other information. For example, the ship speedcould be multiplied by elapsed time to provide the distance traveledthrough the water from any given starting point.

As can be seen, hereinabove, a relatively simple and compact system hasbeen described which greatly reduces the size and complexity of thestandard EM log equipment while maintaining a high degree of accuracyand resolution. Also, the data produced by the system is readily usablein other data processing equipment.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A device for measuring the speed of a ship through water, comprising:

an A.C. source;

a regulator connected to said A.C. source;

said regulator including a feedback network having a voltage variableresistor, said voltage variable resistor being responsive to the outputof said amplifier for controlling the gain of said amplifier and holdingit to a regulated value;

means for generating a magnetic field, connected to the output of saidamplifier, for generating a magnetic field in the water and a voltagecorresponding to said magnetic field and responsive to the speed of thefield through the water;

a sensor within said magnetic field for detecting said voltage;

amplifying means connected to said sensor for amplifying said sensedsignals;

a phasing network connected to said A.C. source for adjusting a phase ofsaid A.C. source to be in phase with the voltage from the sensor;

a ring demodulator;

said phasing network having its output connected to the ring demodulatorand injecting a carrier into said ring demodulator for demodulating saidsensor signal;

said sensor amplifier output being connected to the input of saiddemodulator;

said ring demodulator rectifying the sensor signal and producing anoutput indicative of the speed of the magnetic field through the water.

2. The device of claim 1 wherein said sensor means are pick-up buttonswithin said magnetic field for sensing the voltage and producing asignal from said voltage.

1. A device for measuring the speed of a ship through water, comprising:an A.C. source; a regulator connected to said A.C. source; saidregulator including a feedback network having a voltage variableresistor, said voltage variable resistor being responsive to the outputof said amplifier for controlling the gain of said amplifier and holdingit to a regulated value; means for generating a magnetic field,connected to the output of said amplifier, for generating a magneticfield in the water and a voltage corresponding to said magnetic fieldand responsive to the speed of the field through the water; a sensorwithin said magnetic field for detecting said voltage; amplifying meansconnected to said sensor for amplifying said sensed signals; a phasingnetwork connected to said A.C. source for adjusting a phase of said A.C.source to be in phase with the voltage from the sensor; a ringdemodulator; said phasing network having its output connected to thering demodulator and injecting a carrier into said ring demodulator fordemodulating said sensor signal; said sensor amplifier output beingconnected to the input of said demodulator; said ring demodulatorrectifying the sensor signal and producing an output indicative of thespeed of the magnetic field through the water.
 2. The device of claim 1wherein said sensor means are pick-up buttons within said magnetic fieldfor sensing the voltage and producing a signal from said voltage.